MIMO Explained: With an Interesting Application

Multiple-input, multiple output (MIMO) concepts were pioneered by the cell phone community to minimize the black spots caused by shadows in built-up areas. This technique uses differently located transmitters, each relaying the same message at the same frequency but with each transmission coded with a different orthogonal code, typically orthogonal frequency-division multiplexing (OFDM). A receiver can then decode the incoming signals into separate channels, corresponding to separate signals from various transmitters. The strongest is then selected, hopping from one channel to the other through fades if the platform is moving. Without the orthogonal coding, the various transmitter signals would mutually interfere with each other, sometimes adding, sometimes canceling to cause such fading. These coding ideas have intrigued the radar community, and several forms have been identified with applications quite different from cell phones. Interestingly. these ideas for radar were investigated by the underwater sonar community many years ago but were not termed MIMO.